By: Wesley Holmes

Value: 1 Point

Intent- To limit disruption and pollution of natural water flows by managing stormwater runoff.

Multiple Sources of Stormwater Pollution

Stormwater runoff is a rush, or more appropriately a flush of water that is capable of conveying large quantities of contaminants to waterbodies in a short amount of time. Storm events are typically discrete and relatively short (hours to days) and produce exposures to contaminants potentially lasting from only minutes to hours. Pollutants, including sediment, trash and construction debris from development sites are picked up and washed into receiving streams and other aquatic resources during storm events. Pollutants that accumulate on impervious surfaces (auto fluids, chemical spills) and on compacted pervious surfaces, such as lawns, parks and athletic fields (Pesticide, fertilizer, pet waste) during dry weather are picked up and transported into receiving waters during rainfall events.

In addition to gathering pollutants as it moves  through the developed landscape, stormwater runoff will also pick up a less obvious but still harmful additive, heat. Impervious surfaces, such as rooftops, roads and parking lots, tend to retain heat when exposed to sunlight. This is what is known as a heat island effect. As stormwater runoff moves over these impervious surfaces it absorbs the radiant heat and increases water temperature. Studies conducted by the Center for Watershed Protection and others have found that when this heated stormwater is conveyed into a river, stream, wetland or other aquatic resource, it can decrease the amount of dissolved oxygen contained within the water column, which reduces the amount of oxygen that is available to aquatic organisms causing damage even death to some.

To help prevent the flow of contaminants picked up by stormwater from reaching valuable natural habitats the USGBC requires that projects seeking certification implement a stormwater management plan that captures and treats the stormwater runoff from 90% of the average annual rainfall. The Best Management Practices (BMP’s) utilized for treatment must be designed in accordance with standards and specifications from a state or local stormwater management plan which is capable of removing 80% of the average annual post development total suspended solids (TSS) load (Dirt and other waste). For the project in Charles Town APUS is utilizing a BMP pond, designed by the Civil Engineering firm Dewberry, to capture and treat runoff from the site. Given that the project site is a capped Brownfield, this treatment pond plays a crucial role in containing and treating the runoff from the impervious site cap.

In a typical stormwater management (SWM) pond, a flow regulator is attached to the ponds drain pipe to reduce the size of the outlet. A smaller outlet causes the pond to collect excess flow and reduces the erosion potential of the initial pulse of water. These conventional stormwater retention ponds will release stormwater over 2-3 hours. In a BMP pond the flow regulator attached to the end of the pipe is even smaller. The smaller outlet forces the pond to hold the water for a longer period, allowing more time for the sediment and attached nutrients to settle out in a collection area. Whereas a conventional SWM pond will release stormwater over 2-3 hours, a stormwater management BMP pond may release the water over 2-3 day days allowing for optimum removal of sediment and nutrient loads.

The treatment approach APUS has selected will not only maintain the pre-development rate of runoff from the landscape, it will reduce the impact of the water on receiving ecosystems. This approach complies with the BMP’s of the state and the USGBC and will aide in improving water quality for the Ranson and Charles Town communities.

Images Provided by EPA Watersheds Site

Links of Interest

EPA Urban Stormwater BMP Performance Tool

EPA National Menu of Stormwater Best Management Practices

Center for Watershed Protection

West Virginia Stormwater Management Plan

By: Wesley Holmes

SS Credit 6.1: Stormwater Design—Quantity Control

1 Point

Intent

To limit disruption of natural hydrology by reducing impervious cover, increasing on-site infiltration, reducing or eliminating pollution from stormwater runoff and eliminating contaminants.

Requirements

Case 1-Option 1. For sites with existing imperviousness 50% or less implement a stormwater management plan that prevents the post development peak discharge rate and quantity from exceeding the predevelopment peak discharge rate and quantity for the 1- and 2-year 24-hour design storms.

When land is altered for development, the way stormwater moves through the landscape is fundamentally altered. In an undeveloped landscape, trees, shrubs and other vegetation reduce stormwater runoff volumes through various processes. When rain falls to the ground the leaves of branches and shrubs catch or intercept the rain. This process, termed interception, lowers the total amount hitting the ground and reduces impact on topsoil. Transpiration is a more complex process where water captured in root systems is processed through the plant and released through leaves. In addition, the topography of an area usually will have natural depressions which collect water allowing evaporation. Ultimately these processes work to reduce the amount of water flowing through collecting streams and storm drains. According to a 2008 report issued by the National Research Council on Urban Stormwater Management in the United States in addition to entrainment of chemical and microbial contaminants as stormwater runs over roads, rooftops, and compacted land, stormwater discharge poses a physical hazard to aquatic habitats and stream function, owing to the increase in water velocity and volume that inevitably result on a watershed scale as many individually managed sources are combined.

When land is typically prepared for development it is graded, compacted and in many places paved. Grading removes the native soils and natural depression areas that once worked to retain rainfall and stormwater runoff on site. Compaction reduces the infiltration capacity of the underlying soils and increases the amount of rainfall that is converted to stormwater runoff. The addition of roads, parking lots, rooftops and other impervious surfaces work to further increase stormwater runoff volumes and flow. In the end, much of the rainfall that was once retained in the landscape is now converted to a flow of stormwater runoff. This increase in flow rates can have significant erosion impacts on receiving streams and riparian vegetation.

LEED Site Selection Credit 6.1 requires that the peak discharge rate and quantity of stormwater after development does not exceed the predevelopment peak discharge rate and quantity for the 1- and 2-year 24-hr design storm (Really heavy and lasting rain). Essentially this requires that the Academic Center development to maintain the rate and amount of runoff from the site. The runoff control is being achieved through a retention and treatment pond which will be covered in next weeks post on stormwater quality control.

External Links of Interest

EPA Urban Stormwater BMP Performance Tool

EPA National Menu of Stormwater Best Management Practices

Center for Watershed Protection